iPhone 6s: 'Call Failed' message when making calls

The "Call Failed" issue typically applies to outgoing calls because it occurs when your phone attempts to establish a connection to the network and initiate a call. Incoming calls, on the other hand, are reliant on the network's ability to route the call to your phone, which is a different process.

It seems your iPhone is experiencing call connectivity problems, causing calls to abruptly end with "Call Back" or "Cancel" options on the screen, refer to the pics below. This happens when a call is failed which you forgot to mention in your post.

If you are experiencing intermittent silence or drop or the call itself is disconnected during voice calls on your iPhone, there are a few things you can try to fix the issue:

1. Poor signal strength can cause dropouts or silence during a call. Try moving to an area with better reception or switching to a different network if possible.
2. Restart your iPhone --> Restart your iPhone, iPad, or iPod touch - Apple Support
3. Force Restart your iPhone --> Force restart iPhone - Apple Support)
4. This may help too. General Troubleshooting iPhone Issues: St… - Apple Community
5. Check if your audio is not connected to external Bluetooth speakers.
6. Few other reasons why your device keeps Muting / Dropping / Disconnecting Calls or encountering Call Failed.
   1. Your device's cellular signal Is weak.
   2. It's Raining (Cats & Dogs) in your area or in the area where your device is hooked.
   3. The Cell Tower is Overloaded / Heavy Traffic (Telecommunication)
   4. You've misunderstood your signal strength
   5. You are moving and the signal weakens while changing from one tower to another
7. Other culprits may include the following:
   1. Bad Weather: Precipitation can leak into phone cable lines and cause disruptions. Lightening, even at a distance, can disrupt the connection.
   2. Wireless interference: Power lines, radio, or television. transmitters, transformers, and other electronic devices may cause noise on the phone line. --> Resolve Wi-Fi and Bluetooth issues caused by wireless ...
   3. DSL services: The high-frequency signals coming from DSL (digital subscriber line) devices sharing the phone line can create problems for other technology connected to the line.
8. Try to turn on Airplane mode, wait for 10-15 secs, and turn off aeroplane mode.
9. Check the health of your device's Battery. Change if below 80%
10. Take the SIM out and Put it back again, Change your SIM, if it is damaged
11. Time to contact your Carrier / Change if required